This invention relates generally to an aqueous ink composition containing a colorant which is soluble as a salt, but is relatively water insoluble in its acid form. In particular, the colorant is a polyoxyalkylene substituted organic chromophore which has been reacted with a cyclic acid anhydride, dicarboxylic acid or derivative thereof, to provide a pendant carboxyl group.
Polyoxyalkylene substituted organic chromophores are well known in the art as exemplified by Brendle, U.S. Pat. No. 4,167,510 (fugitive tints), Baumgartner et al., U.S. Pat. No. 4,732,570 (colorants for thermoplastic resins) and Cross et al., U.S. Pat. No. 4,284,729 (colorants reactive into polyurethane resins). Additionally, fugitive colorants within the aforementioned class have been employed in children's marking pens due to their water solubility, which allows for easy removal of the colorant from skin, clothes, painted surfaces, etc. In many ink applications, however, it is essential that the colorant in the ink composition remain permanently affixed to the substrate to which it is applied.
The water solubility of dyestuffs may also be enhanced by grafting such dyestuffs to a water soluble polymer backbone as disclosed in Hugl et al., U.S. Pat. No. 5,030,697. Suitable polymer backbones include co-polymers of a acrylamide with unsaturated dicarboxylic acid anhydrides. Hugl et al. also provide a DNA or antigen reactive group grafted on to the polymer for linking the polymer bound dye to a biologically active material.
The gravure and flexographic printing industries are important areas of ink application requiring a high degree of integrity and permanence of printed images. Typical processes are based upon compositions containing a volatile organic solvent, such as toluene, a resin binder and a pigment or dye to impart color. After the ink composition is applied to a substrate, the solvent is evaporated or absorbed, and the pigment or dye is affixed to the substrate by the binder. Concerns for the environment and limiting worker exposure to organic solvents have increased pressure on the printing industry to develop aqueous based ink compositions. Despite the availability of a number of water soluble dyes and other colorants, such colorants remain essentially water soluble after application and, therefore, lack the permanence found in organic solvent based ink compositions.